Mark Hoeltzel, 46, a pediatric rheumatology specialist whose state medical license was suspended Dec. 21, was having sex with a “vulnerable” female patient in his clinic, gave her pain pills that she didn't need and talked to her about his other teenage patients' "hot" bodies, federal prosecutors allege.

Hoeltzel, who had been working in the University of Michigan's Pediatric Rheumatology Clinic in Ann Arbor, had a two-year sexual relationship with the female patient, referred to him when she was 17 for treatment of rheumatoid arthritis. She also had mental-health diagnoses, according to a criminal complaint unsealed Tuesday in U.S. District Court here.

Hoeltzel is married with children and was arrested Monday at Detroit Metro Airport after returning from an out-of-state trip. Agents were inside the airport waiting to arrest him as he came off the plane even though weeks earlier he had agreed to surrender, according to his lawyer.

Federal authorities learned of the alleged relationship from state licensing authorities, who alerted law enforcement in December that they were investigating Hoeltzel for having a sexual relationship with a female patient.

During the subsequent investigation, law-enforcement officials seized a computer thumb drive from Hoeltzel's home on which they discovered child pornography, according to court records.

Hoeltzel's lawyer, Raymond Cassar of Farmington Hills, Mich., stressed that his client has been charged only with possessing child pornography. As for the claims he was sexually involved with a patient, Cassar said:

"I think there's going to be much more uncovered about this young lady that will balance this out a little bit," Cassar said. "It sounds to me like it was a consensual relationship, and she was of age. She was 18 years old."

Cassar said he also believes this case is limited to one patient.

"This is totally different from the Nassar case," Cassar said, referring to notorious Michigan State University doctor Larry Nassar now serving decades in prison for sexually abusing more than 150 female athletes under the guise of medical treatment.

In the University of Michigan case, the patient said her sexual relationship with the accused doctor started when she was 18.

Hoeltzel texted her after seeing her in the clinic where she was receiving treatment from another doctor and instructed her to make an appointment with him. At her first appointment, Hoeltzel "grabbed her from behind, pulled her butt into his groin and rubbed his erect penis on her," the complaint states.

Over the next two years, the sexual relationship continued — with Hoeltzel visiting the patient at her apartment and having sex with her there, agent Michael MacBride of the Department of Homeland Security Investigations unit wrote in a Jan. 17 affidavit.

The doctor “was seeing the patient in his clinic every three to four weeks and not the usual every three months, which would be normal for her condition and diagnoses,” wrote MacBride, who as a Homeland Security Investigations agent investigates child pornography and child sexual exploitation crimes.

On many occasions, Hoeltzel had sex with her at his clinic and didn’t provide treatment for her arthritis, the patient said in the complaint. He also gave her pain medications “that were beyond and outside of recommendations” for her diagnoses.

During their relationship, Hoeltzel also talked to the patient about his 12- to 15-year-old female patients, whom he described as "hot," according to the complaint.

The University of Michigan fired Hoeltzel as a result of a State of Michigan Licensing and Regulatory Affairs investigation. The university officials said they are cooperating with authorities and have "retained an experienced national law firm with expertise in this area to investigate this matter."

"These are very disturbing and serious allegations, and we have reached out to our patients to inform them of concerns related to Dr. Hoeltzel, offer resources and provide them with a way to report any concerns," Marschall Runge, chief executive of Michigan Medicine and the university's executive vice president of medical affairs, said in a statement Monday.

After learning in early December that the state licensing board was investigating Hoeltzel for alleged sexual misconduct involving a patient, the University of Michigan acted quickly, according to Runge.

"We took immediate steps to protect our patients by removing him from patient care duties the day we were alerted. We did not allow him to return to work, and he is no longer a Michigan Medicine employee," Runge said in the statement. "We also immediately reported him to law enforcement."

He continued: "Any behaviors we discover that risk the safety of our patients or employees or violate state or federal law will be reviewed and addressed with the appropriate follow up."

On Dec. 11, a search warrant was issued for Hoeltzel’s Ann Arbor home to further an investigation into “potential criminal conduct, including criminal sexual conduct first degree,” according to court documents. On Dec. 12, law enforcement officers searched his home and seized various electronic media, including a flash drive.

A detective “found child pornography in plain view on the device,” the complaint states. The flash drive had 210 images of child pornography, most of them minor females with their genitals exposed. The files were created in 2008 and were last accessed May 17, 2017, the complaint states.

Hoeltzel’s laptop also was seized. The University of Michigan Police Department is conducting the forensic examination of the laptop.

Those results are not yet known. Hoeltzel is charged with possessing and receiving child pornography.

His detention hearing is scheduled for 1 p.m. ET Wednesday when a judge will decide whether to release him on bond or keep him locked up pending the outcome of his case. The government is seeking to keep him jailed.